Backups would not work after a migration from RHEL -> Ubuntu 8.04. Running 'zmbackup -f -a all' would result in a 3.3MB backup of what seemed like just ldap data. Accessing the backup section in the GUI Admin resulted in the above error message.

Fix:

Running strace on zmbackup, the script was trying to access /opt/zimbra/lib/ext/backup/zimbrabackup.jar, which did not exist. The fix was to unpack the zimbra-store deb and copy opt/zimbra/lib/ext/backup into place.